This invention relates to a method of measuring the displacement of the optical axis of an optical microscope that is used in the lithography process for manufacturing semiconductor devices, an optical microscope and an evaluation mark. More particularly, the present invention relates to a method of measuring the displacement of the optical axis of the optical microscope being used for an optical alignment device or an alignment gauge for the purpose of regulating the microscope as well as to an optical microscope and an evaluation mark that can effectively be used with the method.
The process of manufacturing semiconductor devices typically includes a so-called lithography step where the pattern formed on the photomask is transferred onto the resist film applied to the surface of a silicon wafer by means of a demagnification projection aligner and another lithography step that is conducted by using another photomask aligned relative to the transferred pattern after carrying out an etching, ion implanting or film forming operation. Then, the above described sequence of operation is repeated for more than ten times. Meanwhile, semiconductor engineers are paying efforts for designing downsized microchips of semiconductor devices that can reduce the manufacturing cost in terms of each and every function of the device and also show an improved performance. However, since the required accuracy of alignment in the lithography step is ⅓ to xc2xc of the minimum line width, downsized microchips of semiconductor devices have to meet rigorous requirements in terms of accuracy of alignment.
Because various factors affect the accuracy of alignment, efforts have to be paid in a concerted manner for the purpose of improving the accuracy. For instance, in the aligner, the precision of the mask stage and the wafer stage has to be improved and the distortion of the demagnification objectives needs to be reduced, while the precision of the alignment device (to be referred to as alignment sensor hereinafter) has also to be improved. In line with these efforts, the flow of air and the propagation of heat in the aligner are being analytically studied.
As for designing and manufacturing semiconductor devices, alignment marks are designed in an ingenious way so as to accommodate the influence of the manufacturing process. Particularly, the planarization of wafer substrates by means of the chemical/mechanical polishing (CMP) technology developed in recent years gives rise to a reduced level of detection signal and asymmetry of alignment marks so that measures for coping with the CMP technology have to be provided when designing alignment marks.
As for alignment gauges for determining the accuracy of alignment (to be referred to as overlay inspection tool hereinafter), efforts are also being paid to improve the accuracy of alignment and reduce the influence of fluctuations in the manufacturing process.
The alignment sensors and the overlay inspection tools that are currently being used mostly comprise an optical magnification microscope as basic component, which is used to pickup a magnified image of the alignment mark formed on the wafer substrate and that of the alignment check mark (to be referred to as bar-in-bar mark hereinafter) by means of a CCD (Charge-Coupled Device) camera. Then, the image data obtained from the picked up images are processed to determine the positions of the marks and the accuracy of alignment thereof. Therefore, the performance of the optical magnification microscope mounted on an alignment sensor or an overlay inspection tool is directly reflected to the accuracy of measurement of the instrument. Thus, the technique and the mechanism for regulating such a microscope need to be of a level much higher than those being used for regulating ordinary optical microscopes.
In view of these circumstances, a number of methods for evaluating the microscope of an alignment sensor or an overlay inspection tool have been proposed. For instance, Jpn. J. Appl. Phys. 36 (1997), pp. 7512-7516 (Reference Document 1) describes a method for observing the asymmetry of the signal representing a pair of steps of a groove cut into a silicon wafer. This method is currently most popularly used to optically regulate microscopes of the type under consideration. Jpn. Pat. Appln. KOKAI No. 10-287837 (Reference Document 2) describes an evaluation method based on the coma of the microscope of the alignment sensor. With this method, wide patterns and narrow patterns are alternately arranged on a silicon wafer at regular intervals and the intervals are observed by means of an alignment sensor.
Therefore, it is the object of the present invention to provide a simple method of measuring the displacement of the optical axis of an optical microscope that is used in the lithography process for manufacturing semiconductor devices as well as an optical microscope and an evaluation mark that can effectively be used with such a method.
According to the invention, the above object is achieved by providing a method of measuring the displacement of the optical axis of an optical microscope having an illumination optical system and a projection optical system, the method having:
a step of irradiating the evaluation mark having diffraction grating patterns formed on a substrate with illumination light by way of the illumination optical system and observing the evaluation mark by way of the projection optical system to obtain the brightness of the evaluation mark; and
a step of measuring the displacement of the optical axis on the basis of the relationship between the brightness of the image of the evaluation mark and the direction of the diffraction grating patterns of the evaluation mark.
Since the brightness of the obtained image of the evaluation mark varies as a function of the direction of the diffraction grating patterns and the variance becomes very remarkable when the optical axis is displaced to a large extent. Therefore, measuring the brightness of the image of the evaluation mark is equivalent to measuring the displacement of the optical axis. In other words, the displacement of the optical axis, if any, can be easily detected and measured by observing the brightness of the image of the evaluation mark.
The displacement of the optical axis is evaluated in terms of the direction (for example as expressed by x and y of an x-y orthogonal coordinate system ) of the diffraction grating patterns where the brightness of the image of the evaluation mark is maximal. While evaluation mark may be observed for a plurality of times, changing the direction of the substrate and hence that of the evaluation mark, to compare the difference in the brightness among the obtained images of the evaluation mark, the number of observations can be reduced by using an evaluation mark having two or more than two diffraction grating patterns that are arranged in different directions.
The brightness of the image of the evaluation mark has to be differentiated to a large extent in order to improve the sensitivity of observation. This can be realized effectively by block normal light at the position of the pupil of the projection optical system or by blocking a plurality of diffracted beams of light produced by the diffraction grating patterns relative to normal light at the position of the pupil of the projection optical system. An optical microscope according to the invention is devised to realize the latter technique.
The displacement of the optical axis can be evaluated in terms of direction (as expressed by +x or xe2x88x92x and +y or xe2x88x92y of an orthogonal coordinate system) by using a plurality of beams of diffracted light that are produced by the diffraction grating patterns and whose intensities vary asymmetrically relative to normal light.
The above object and other objects as well as the novel features of the present invention will become clear from the following description made by referring to the accompanying drawing.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.